Thin waveguide wavelength-selective projector

ABSTRACT

A device for providing a 1D line of an image is disclosed. The device includes a wavelength-tunable light source for providing image light having the angular distribution encoded in optical spectrum. The device further includes a thin slab waveguide having an out-coupler in form of a diffraction grating for out-coupling the image light at an angle dependent on wavelength. The image may be formed by scanning a collimated beam propagating in the slab waveguide when using tunable monochromatic light sources, or by forming the 1D singular distribution of brightness at a same time when using a tunable-spectrum light sources. The device may be used in a near-eye display for forming a 2D image in angular domain.

REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. Provisional PatentApplication No. 63/002,817, filed on Mar. 31, 2020 and entitled “Single-or Few-Mode Waveguide Display” and U.S. Provisional Patent ApplicationNo. 63/012,625, filed on Apr. 20, 2020 and entitled “Single- or Few-ModeWaveguide Display”, both of which being incorporated herein by referencein their entireties.

TECHNICAL FIELD

The present disclosure relates to optical devices, and in particular todisplay systems and modules.

BACKGROUND

Head mounted displays (HMD), helmet mounted displays, near-eye displays(NED), and the like are being used increasingly for displaying virtualreality (VR) content, augmented reality (AR) content, mixed reality (MR)content, etc. Such displays are finding applications in diverse fieldsincluding entertainment, education, training and biomedical science, toname just a few examples. The displayed VR/AR/MR content can bethree-dimensional (3D) to enhance the experience and to match virtualobjects to real objects observed by the user.

To provide better optical performance, display systems and modules mayinclude a large number of components such as lenses, waveguides, displaypanels, etc. Because a display of an HMD or NED is usually worn on thehead of a user, a large, bulky, unbalanced, and/or heavy display devicewould be cumbersome and may be uncomfortable for the user to wear.Compact, lightweight, and efficient head-mounted display devices andmodules are desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1A is a 3D view of a device for redirecting light in twodimensions, the device including a low-mode waveguide;

FIG. 1B is a side cross-sectional view of the low-mode waveguide of FIG.1A;

FIG. 1C is a schematic diagram illustrating rendering of an image by thedevice of FIG. 1A;

FIG. 2 is a system-level block diagram of an embodiment of the device ofFIGS. 1A and 1B;

FIG. 3 is a schematic frontal view of a near-eye display embodiment ofthe device of FIG. 2;

FIG. 4A is a schematic frontal view of a photonic integrated circuit(PIC) embodiment of the near-eye display of FIG. 3;

FIGS. 4B and 4C are side and top views, respectively, of a personwearing the near-eye display device of FIG. 4A;

FIG. 5 is a schematic side view of a wavelength-tunable light sourceincluding an intracavity spectrally selective element;

FIG. 6A is a schematic side view of a tunable-spectrum light sourceincluding an external dynamic spectrally selective element;

FIG. 6B is an output spectrum of the source of light superimposed with atransmission spectrum of the spectrally selective element, forillustrating the principle of operation of the tunable spectrum lightsource of FIG. 6A;

FIG. 6C is an output spectrum of the light source of FIG. 6A;

FIG. 7 is a side cross-sectional view of a free-space grating couplerfor coupling light into a waveguide;

FIG. 8 is a side cross-sectional view of an adiabatic waveguide couplerfor coupling light into a waveguide;

FIG. 9 is a schematic view of a phased array 1D imager of thisdisclosure;

FIG. 10A is a schematic top view of a PIC implementation of the phasedarray 1D imager of FIG. 9;

FIG. 10B is a schematic top view of a PIC implementation of aMach-Zehnder interferometer (MZI) array (MZIA) of the PIC 1D imager ofFIG. 10A;

FIG. 10C is a schematic top view of an individual MZI of FIG. 10B;

FIG. 10D is a schematic top view of a phase shifter array (PSA) of thePIC 1D imager of FIG. 10A;

FIG. 10E is a schematic top view of an implementation of a waveguide fanof the PIC 1D imager of FIG. 10A;

FIG. 11 is a schematic top view of a hybrid 1D imager including 1Dlenses etched in the waveguide;

FIG. 12 is a schematic view of a free-space optics (FSO) implementationof a 1D scanner including a microelectromechanical system (MEMS)tiltable reflector;

FIG. 13A is a top schematic view of a field of view (FOV) expander basedon liquid crystal (LC) cladding waveguides;

FIG. 13B is a side cross-sectional view of the FOV expander of FIG. 13A;

FIG. 14 is a schematic top view of a hologram-based beam expander inaccordance with this disclosure;

FIG. 15 is a schematic top view of a PIC implementation of a phasedarray 1D imager with a beam expander;

FIG. 16 is a side cross-sectional view of an output waveguide forillustration of angular dispersion calculation;

FIG. 17 is a side cross-sectional view of a dual-core waveguide forbroadening an angular dispersion range of an out-coupler;

FIG. 18 is a top view of a few-modes waveguide (FMW) for broadening anangular dispersion range of an out-coupler;

FIG. 19 a side cross-sectional view of a high-dispersion out-couplerbased on a slow light waveguide;

FIG. 20A is a side cross-sectional view of a high-dispersion out-couplerbased on a waveguide grating and a corrugated reflector;

FIG. 20B is a side cross-sectional view of a high-dispersion out-couplerbased on a waveguide grating and a polarization-selective corrugatedreflector;

FIGS. 21A and 21B are schematic side views of a low-mode slab waveguidewith a gradient of refractive index for variably focusing or defocusingthe image light out-coupled from the waveguide;

FIG. 22 is a side cross-sectional views of a varifocal out-couplerembodiment based on a tilted liquid crystal (LC) cell;

FIG. 23 is a side cross-sectional views of a varifocal out-couplerembodiment based on a wedged Pockels cell;

FIG. 24 is a side cross-sectional views of a varifocal out-couplerembodiment based on a Pockels cell with buried electrodes;

FIG. 25 is a side cross-sectional view of a varifocal out-couplerembodiment based on a thermo-optic effect;

FIG. 26 is a flow chart of method for providing an image in angulardomain;

FIG. 27 is a flow chart of a variant of the method of FIG. 26 utilizing1D scanning/rendering at a set wavelength;

FIG. 28 is a flow chart of a variant of the method of FIG. 26 usingsimultaneous generation of frames of an image to be displayed;

FIG. 29 is a view of an augmented reality (AR) display of thisdisclosure having a form factor of a pair of eyeglasses; and

FIG. 30 is an isometric view of a virtual reality (VR) display of thisdisclosure, according to an embodiment.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. In FIGS.1A, 1B, 2, 3, 4A, 5, 6A, 7-9, 10A-10E, 11, 12, 13A, 13B, and 14-25,similar reference numerals denote similar elements.

Near-eye displays (NEDs) may use pupil-replicating waveguides to expanda projected image over an eyebox of the display, i.e. over an area whereuser's eye may be located during normal operation, e.g. when the displayis worn by the user. A pupil-replicating waveguide is typically aparallel slab of a transparent material propagating image light in azigzag pattern by total internal reflection (TIR) from the waveguide'stop and bottom surfaces. Such waveguides may be prone to diffractioneffects that cause color dispersion as function of field angle, and aregenerally unsuitable for curved substrates and present ghost images forreal-world up-close objects.

In accordance with this disclosure, single mode (SM) or few-mode (FM)waveguides, termed herein collectively as “low-mode” waveguides, may beused to deliver light to the eyebox and form an image. An advantage oflow-mode waveguides is that light interacts with gratings several ordersof magnitude more often compared to a regular multimode light guide. Asa result, the diffraction efficiency of the grating for every singleinteraction may be made small enough to lessen or eliminate see-throughartifacts such as rainbow and improve conspicuity of the display.Additionally, single mode waveguides enable more precise control oflight distribution across the eyebox, which leads to better uniformityand efficiency.

A challenge of using a single mode waveguide is that it only hascapacity to transmit 1D information, e.g. horizontal resolution but notvertical resolution, or vice versa. This limitation may be overcome byencoding the other component of 2D image in a non-spatial characteristicof light such as wavelength, for example. The wavelength range used foreach color channel may be made small enough to not reduce the colorgamut significantly.

In accordance with this disclosure, there is provided device forproviding a line of an image in angular domain. The device includes awavelength-tunable light source for providing image light comprising aspectral component at a first wavelength, and a low-mode waveguide. Thelow-mode slab waveguide includes an in-coupler for coupling the imagelight into the low-mode waveguide, and a slab waveguide portion forpropagating the image light coupled by the in-coupler. The slabwaveguide portion includes an out-coupler configured to out-couple thespectral component of the image light at an angle to a plane of the slabwaveguide portion, the angle being dependent on the first wavelength.The slab waveguide portion may include a singlemode slab waveguide. Insome embodiments, the slab waveguide portion includes a core and a topcladding supported by the core. The in-coupler may include a diffractiongrating formed in the core of the slab waveguide portion.

In some embodiments, the slab waveguide portion includes a first coreand a first cladding supported by the first core. The first core and thefirst cladding may be configured for singlemode propagation of the imagelight, the out-coupler comprising a first diffraction grating formed inthe first core. The first diffraction grating may be configured toout-couple the spectral component of the redirected image light at afirst angle dependent on the first wavelength. The first angle is withina first angle range corresponding to a tuning range of thewavelength-tunable light source. The slab waveguide portion may furtherinclude a second core supported by the first cladding and a secondcladding supported by the second core. The second core and the secondcladding may be configured for singlemode propagation of the imagelight. The out-coupler may further include a second diffraction gratingformed in the second core. The second diffraction grating may beconfigured to out-couple the spectral component of the redirected imagelight at a second angle different from the first angle. The second angleis within a second angle range corresponding to the tuning range of thewavelength-tunable light source. The second range is different from thefirst range.

A multimode interference (MMI) coupler may be disposed in an opticalpath between the in-coupler and the first and second cores of the slabwaveguide portion, for coupling the image light into at least one of thefirst and second cores of the slab waveguide portion. In suchembodiments, the device may further include a 1×2 optical switch and avertical mode converter downstream of the 1×2 optical switch in anoptical path between the in-coupler and the MMI coupler. An input portof the 1×2 optical switch may be coupled to the in-coupler, and firstand second output ports of the 1×2 optical switch may be coupled tofirst and second input ports, respectively, of the vertical modeconverter. The vertical mode converter may be configured to couple lightat its first input port to the first core of the slab waveguide portion,and couple light at its second input port to the second core of the slabwaveguide portion.

In some embodiments, the device further includes a focusing gratingsupported by the slab waveguide portion. The focusing grating mayinclude an array of grating fringes having a first refractive index, anda substrate between individual fringes of the array of grating fringes,the substrate having a second refractive index. At least one of thefirst or second refractive index may be tunable to provide a gradient ofthe at least one of the first or second refractive index for focusing ordefocusing of the image light out-coupled from the slab waveguideportion by the out-coupler. The substrate may include liquid crystals. Aspatially selective heater may be coupled to the focusing grating andconfigured to create the temperature gradient across the focusinggrating to provide a gradient of the at least one of the first or secondrefractive index.

In some embodiments, the slab waveguide portion comprises a photoniccrystal slab layer supporting a cladding layer and having a group indexof at least 10. The out-coupler may include a diffraction gratingsupported by the cladding layer for out-coupling the image lightpropagating in the photonic crystal slab layer.

In some embodiments, the slab waveguide portion comprises a few-modeslab waveguide supporting no more than 10 lateral modes of propagation,the few-mode slab waveguide comprising a core. The out-coupler mayinclude a diffraction grating formed in or on the core. The diffractiongrating may be configured to out-couple the spectral component of theredirected image light at an angle dependent on the first wavelength.The angle may be within an angle range corresponding to a tuning rangeof the wavelength-tunable light source. The angle range is different fordifferent lateral modes of propagation of the few-mode slab waveguide.

An MMI coupler may be provided in an optical path between the in-couplerand the few-mode slab waveguide portion, for coupling the image lightinto at least one mode of propagation of the few-mode slab waveguide.The device may further include a 1×N optical switch and a vertical modeconverter downstream of the 1×N optical switch in an optical pathbetween the in-coupler and the MMI coupler. An input port of the 1×Noptical switch may be coupled to the in-coupler, and N output ports ofthe 1×N optical switch may each be coupled to a particular one of Ninput ports, respectively, of the vertical mode converter, where N is aninteger. The vertical mode converter may be configured to couple lightreceived at its input ports to a corresponding mode of propagation ofthe few-mode slab waveguide portion.

In some embodiments, the out-coupler comprises a diffraction gratingconfigured to out-couple the spectral component of the image light at anangle exceeding 90 degrees w.r.t. a direction of propagation of theimage light in the slab waveguide portion. In such embodiments, thedevice may further include a corrugated reflector supported by the slabwaveguide portion for reflecting the image light diffracted by thediffraction grating through the slab waveguide portion and outside ofthe low-mode waveguide. The corrugated reflector may include apolarization-selective reflector configured to reflect light at a firstpolarization and transmit light at a second polarization orthogonal tothe first polarization. In such embodiments, the device may furtherinclude a quarter-wave waveplate (QWP) supported by the slab waveguideportion on an opposite side of the slab waveguide portion from thepolarization-selective reflector, and configured for receiving the imagelight reflected by the polarization-selective reflector, the image lighthaving the first polarization; and a diffractive structure supported bythe quarter-wave waveplate and configured to reflect the image lightpropagated through the QWP back to propagate through the QWP for asecond time converting the polarization of the image light to the secondpolarization, through the slab waveguide portion, and through thepolarization-selective reflector.

In accordance with the present disclosure, there is provided a low-modewaveguide comprising a slab waveguide portion for propagating light inthe slab waveguide portion. The slab waveguide portion includes anout-coupler configured to out-couple the light at an angle to a plane ofthe slab waveguide portion, and a liquid crystal (LC) cell evanescentlycoupled to the slab waveguide portion. In operation, the LC cell definesan effective refractive index for the light propagating in the slabwaveguide portion. The effective refractive index varies in a directionof propagation of the light in the low-mode slab waveguide, whereby adirection of the light out-coupled by the out-coupler from the slabwaveguide portion is varied along the a direction of propagation of thelight in the low-mode slab waveguide. The LC cell may form an acuteangle with the slab waveguide portion.

In accordance with the present disclosure, there is further provided alow-mode waveguide comprising a slab waveguide portion for propagatinglight in the slab waveguide portion. The slab waveguide portion mayinclude a core layer and an out-coupler configured to out-couple thelight at an angle to a plane of the slab waveguide portion. The corelayer has a refractive index dependent on an applied electric field. Thelow-mode waveguide may further include electrodes above and below thecore layer, for applying the electric field to the core layer, such thatthe electric field is spatially varying along a direction of propagationof light in the core of the slab waveguide portion. In operation, aneffective refractive index for the light propagating in the slabwaveguide portion varies in the direction of propagation of the light inthe low-mode slab waveguide, whereby a direction of the lightout-coupled from the slab waveguide portion is varied along the adirection of propagation of the light in the low-mode slab waveguide.The electrodes may be disposed at an acute angle to each other.

Referring now to FIG. 1A, a device 100 provides an image in angulardomain. The image may be defined by a light field having atwo-dimensional (2D) angular distribution of brightness I(α,β). Theangles α,β are ray angles of the image light defining the ray angle in3D space, as shown in FIG. 1A. More generally, the device 100 is usablefor redirecting light in two dimensions or along two non-parallel planesor surfaces, e.g. for 2D beam rastering, remote sensing, depth sensing,LIDAR applications, etc. Herein, the term “redirecting” includesrastering of a collimated light beam, as well as providing instantaneousdistributions of light in one or two dimensions or planes.

The device 100 includes a light source 102, an 1D redirector 104 coupledto the light source 102, and a low-mode waveguide 106 coupled to the 1Dredirector 104 by a coupler 103. The low-mode waveguide 106 includes aslab waveguide portion 107. Herein, the term “slab waveguide” denotes awaveguide that limits the light propagation only in one dimension, i.e.vertical direction or Z-direction perpendicular to the waveguide plane,allowing the light to freely propagate in plane of the waveguide, e.g.in XY plane in the example of FIG. 1A. The light source 102 provideslight 108 having a tunable optical spectrum, which is a function of thedesired distribution of brightness I(α,β). For example, the tunableoptical spectrum may have a plurality of spectral components; in someembodiments, one spectral component may be provided at a tunablewavelength.

The 1D redirector 104 receives the light 108 from the light source 102and redirects, e.g. angularly disperses, the light 108 in XY plane, i.e.the plane of the low-mode waveguide 106, in accordance with the desireddistribution of brightness I(α,β). In display applications, the 1Dredirector 104 functions as a 1D imager providing a line of a 2D image.The slab waveguide portion 107 is a singlemode waveguide or a few-modewaveguide configured for propagating the light 108 in XY plane butconfining and guiding the light propagation along Z-axis. The slabwaveguide portion 107 includes an out-coupler 110 that out-couples theimage light 108 at angle(s) to the low-mode waveguide 106 plane (XYplane), the angle depending on the wavelength(s) of the spectralcomponent(s) of the light 108. In some embodiments, a single tunablespectral component may be out-coupled by the out-coupler 110 at an angledepending on its wavelength, in a plane disposed at an angle to theplane of the low-mode waveguide 106, that is, XY plane; and in someembodiments, a plurality of spectral components is out-coupledsimultaneously, or instantaneously, at a distribution of anglescorresponding to the distribution of the wavelengths of the spectralcomponents. The distribution of the out-coupling angles is defined bythe tunable optical spectrum of the light 108. The angular distributionof brightness in both X- and Y-directions may be controlled to providethe image in angular domain having the the desired distribution ofbrightness I(α,β), for direct observation by a viewer.

FIG. 1B shows an embodiment of the low-mode waveguide 106 in a sidecross-sectional view. The low-mode waveguide 106 includes a substrate112 supporting a thin waveguide layer 114 in which the light 108propagates. Depending on refractive index contrast and thickness of thewaveguide layer 114, only one mode or several, e.g. up to 10 modes, maypropagate in the thin waveguide layer 114. Thus, the term “low-mode”waveguide is defined herein to mean a waveguide supporting up to 10different lateral modes of propagation. The light 108 propagates inplane of the waveguide 106 i.e. XY plane, but is constrained, or guided,in Z-direction.

The wavelength-selective out-coupler 110 out-couples the image 108 atdifferent angles depending on wavelength. For example, a first spectral121 component at a wavelength λ₁ is out-coupled at a straight angle tothe low-mode waveguide 106, and a second spectral component 122 at awavelength λ₂ is out-coupled at an acute angle to the low-mode waveguide106. The spectral composition of the light 108 provided by the lightsource 102, as well as the angular dispersion of thewavelength-selective out-coupler 110, are selected so as to provide thedesired angular distribution of brightness I(α,β(λ)). By way of anon-limiting example, referring to FIG. 1C, an entire image 116 inangular domain may be formed. The image 116 is represented by theangular distribution of brightness I(α,β(λ)).

Referring to FIG. 2, a device 200 is an example implementation of thedevice 100 of FIGS. 1A and 1B. The display device 200 of FIG. 2 includesserially optically coupled: a spectrally tunable singlemode light source202, an in-coupler 203, an 1D imager 204, the following optionalmodules: 1D FOV expander 224, 1D lateral beam expander 226, and angulardispersion enhancer 228; and an out-coupler 210. Other elements may alsoinclude a varifocal adjuster 230, a stray light filter 232, and adistributed temperature sensor 234. Optional elements are shown indashed round-corner rectangles. All or some of the elements may be apart of a low-mode waveguide 206, e.g. may be formed in or on thelow-mode waveguide 206. It is to be noted that the low-mode waveguide206 may include a section with linear waveguides, i.e. straight orcurved ridge-type waveguides guiding light in two dimensions, and a slabwaveguide section, guiding light in only one dimension, i.e. Z-directionin FIG. 2, while allowing free propagation in XY plane. The order ofcoupling of the elements shown in FIG. 2 may vary.

In operation, the spectrally tunable singlemode light source 202provides image light 208 having a tunable optical spectrum, which is afunction of the desired angular distribution of brightness I(α,β(λ)) asexplained above. The coupler 203 couples the image light 208 into the 1Dimager 204. The 1D imager 204 receives the image light 208 from thelight source 202 and redirects or angularly disperses the image light208, scans a collimated beam of the image light 208, etc. The 1D FOVexpander 224 may be configured to switch the image light 208 between aplurality of conterminous FOV portions to enhance or broaden the spreadof light. The 1D lateral beam expander 226 increases a width ofcollimated portions of the image light in the XY plane, i.e. broadensthe image light 208 beam in plane of the low-mode waveguide 206 therebyincreasing the lateral size of the eyebox of the display device 200.Herein, the term “eyebox” means a geometrical area where an image ofacceptable quality may be observed by a user of the display device 200.The angular dispersion enhancer 228 increases the spectral dispersion ofthe image light 208, to arrive at the desired second 1D angulardistribution of brightness I(β) when out-coupled by thewavelength-selective out-coupler 210. The varifocal adjuster 230 mayadjust convergence or divergence of the out-coupled image light to varythe perceived depth of focus. The stray light filter 232 may remove orlessen portions of the image light out-coupled not to the eyes of theuser but outwards to the external world. The distributed temperaturesensor 234 may obtain a temperature distribution across the low-modewaveguide 206 to provide corrections and to operate thermally-drivenoptical elements and components. More details will be given below.

Referring to FIG. 3, a display device 300 is an implementation of thedevice 100 of FIG. 1 or the device 200 of FIG. 2. The display device 300of FIG. 3 is a near-eye display device having a form factor of a pair ofeyeglasses 311, with a low-mode waveguide 306 occupying a lens area ofthe eyeglasses. The display device 300 of FIG. 3 includes seriallyoptically coupled: a light source 302, a coupler 303, an 1D imager 304,a 1D FOV expander 324, a 1D lateral beam expander 326, an angulardispersion enhancer 328, an out-coupler 310, and a varifocal adjuster330. All elements may be implemented in the low-mode waveguide 306. Thelight source 302 may be disposed separately, as shown.

Referring to FIG. 4A, a display device 400 is an implementation of thedevice 100 of FIG. 1, the device 200 of FIG. 2, or the display device300 of FIG. 3. The display device 400 of FIG. 4A is a near-eye displaydevice having a form factor of a pair of eyeglasses 411, with activecomponents/features implemented in a PIC portion 436 of a waveguide 406.The PIC portion 436 may be driven by an integrated circuit (IC) driverunit 407. The display device 400 includes a light source 402 having atunable emission spectrum, coupled to the PIC portion 436 via an opticalfiber 403. The PIC portion 436 may include, for example, a 1D imagerand/or a 1D FOV expander. Light beams 408A, 408B, and 408C (collectively408) formed in the LC portion 436 are expanded by in-waveguide opticalelements 426A, 426B, and 426C respectively, having optical power (i.e.focusing/defocusing power) for collimating/redirecting the light beams408A, 408B, and 408C towards a highly dispersive output grating 410. Thefunction of the highly dispersive output grating 410 is to provide theout-coupling of image light 408 at different vertical angles to providea vertical FOV, as illustrated in FIG. 4B. The horizontal FOV shown inFIG. 4C is provided by the PIC portion spreading the image light 408in-plane of the waveguide 406.

Various implementations of different modules depicted in FIGS. 1, 2, 3,and 4A will now be considered.

Referring first to FIG. 5, a tunable laser source 502 may be used as awavelength-tunable light source for a display device of this disclosure.The tunable laser source 502 includes an optical cavity formed by a pairof mirrors 501, a gain medium 504, and a wavelength-selectiveintracavity element 506 having a transmission peak tunable within a gainspectral band of the gain medium 504. The transmission peak of thewavelength-selective element 506 may be scanned in coordination withscanning an 1D angle of a collimated beam of light 508 to provide therequired 2D FOV of the display device, with e.g. horizontal FOV providedby the beam scanning, and vertical FOV provided by the output wavelengthscanning of the tunable laser source 502.

Turning to FIG. 6A, a tunable-spectrum light source 602 may be used as awavelength-tunable light source for a display device of this disclosure.The tunable spectrum light source 600 includes a broadband source 604 oflight 608 coupled to a dynamic spectral filter 606 having a selectablearbitrary spectral shape. Emission spectrum 630 of the broadband source604 is illustrated in FIG. 6B, where it is overlapped with an examplebroadband spectral shape 632 of the dynamic spectral filter 606, or anexample narrowband (single-wavelength) spectral shape 633 of the dynamicspectral filter 606.

Dynamic spectral filter 606 may be configured to independently adjusttransmission of a single or a plurality of adjacent narrow spectralbands or channels. For example, the dynamic spectral filter 606 mayadjust the shape of the broadband spectral shape 632 in accordance withthe desired angular distribution of brightness at the output of thedisplay device. In some embodiments, the narrowband spectral shape 633may be scanned in wavelength, which results in the output light beambeing scanned angularly in accordance with the dispersion function ofthe out-coupler of the display device.

Resulting output spectra are shown in FIG. 6C. For example, thebroadband spectral shape 632 results in a broadband emission spectrum634, and the narrowband spectral shape 633 accordingly results in anarrowband emission spectrum 635, the emission wavelength being tunableby the dynamic spectral filter 606.

Referring to FIG. 7, a free-space grating coupler 703 may be used tocouple light from a tunable-spectrum light source 702A, such as thetunable laser source 502 of FIG. 5 or a tunable-spectrum light source602 of FIG. 6A, into a waveguide or PIC of a display disclosed herein.The free-space grating coupler 703 includes a plurality of grating lines705 that receive image light 708 and couple the image light 708 into acore 707 of a waveguide 706. The grating lines 704 run parallel to eachother and may be straight or curved, e.g. may have shapes of concentricarc sections running perpendicular to the plane of FIG. 7. Theconcentric arc shape provides focusing of the image light 708, matchingits mode size to a size of an optical mode 709 that can propagate in thecore 707 of the waveguide 706.

Turning to FIG. 8, a waveguide coupler 803 may be used to couple lightfrom a tunable-spectrum, waveguide-based or fiber-coupled light sourceinto a waveguide or PIC of a display of this disclosure. The waveguidecoupler 803 includes a tapered section 836 where a tapered core 813 of asource optical fiber 802 is disposed in close proximity and parallel toa tapered waveguide core 805 of a waveguide 806 having a waveguide core807. The tapered section 836 may be long enough to ensure an adiabatictransition of optical energy from the source optical fiber 802 into awaveguide core 807 of the waveguide 806.

Referring to FIG. 9, a phased array 1D imager 904 is an embodiment ofthe 1D redirector/imager 104 of FIG. 1, the 1D imager 204 of FIG. 2, orthe 1D imager 304 of FIG. 3. The phased array 1D imager 904 of FIG. 9includes a 1×N power splitter 920, N phase shifters 922 coupled to thepower splitter 920, and an array of N linear waveguide emitters 924coupled to the phase shifters 922. Throughout this specification, theterm “linear waveguide” denotes a waveguide that bounds the lightpropagation in two dimensions, like a light wire. A linear waveguide maybe straight, curved, etc.; in other words, the term “linear” does notmean a straight waveguide section. One example of a linear waveguide isa ridge-type waveguide. All elements of the phased array 1D imager 904may be implemented in a low-mode waveguide 906 including a slabwaveguide portion 907. The number N may vary between 4 and 16,000, forexample.

In operation, an in-coupler, e.g. the free-space grating coupler 703,receives image light 908 from a wavelength-tunable laser source, notshown in FIG. 9, and couples the image light 908 into the power splitter920. The power splitter 920 distributes the image light 908 between Nlinear waveguides 921 of the low-mode waveguide 906, each linearwaveguide 921 carrying a portion of the image light 908. Each imagelight portion is shifted in phase, or delayed, by a corresponding phaseshifter 922, based on a control signal provided by a controller 926. Theimage light portions are emitted by the array of N linear waveguideemitters 924 with a phase profile corresponding to a desired beam angleϑ, forming an output beam 919 having a phase front 921. The output beam919 propagates in a slab waveguide portion 907. In some embodiments, thephase profile in 922 may be controlled to suppress all diffractionorders but one, such that all energy is concentrated into a singlesteered beam.

Turning to FIG. 10A, a PIC phased array 1D imager 1004 is an exampleimplementation of the phased array 1D imager 904 of FIG. 9. The PICphased array 1D imager 1004 of FIG. 10 includes a Mach-Zehnderinterferometers array (MZIA) 1020 operating as the splitter 920, a PICphase shifter array (PSA) 1022 coupled to the MZIA 1020, and a waveguideconcentrator 1024 coupled to the PIC phase shifter array 1022. Ends ofoutput linear waveguides 1030 of the waveguide concentrator 1024 operateas the antennae 924 (FIG. 9) emitting light that propagates freely inplane of the slab waveguide portion (XY plane in FIG. 10A), whileremaining bound in a direction perpendicular to the slab waveguide(Z-direction in FIG. 10A).

The MZIA 1020 may include a binary tree of passive Y-splitters and/oractive Mach-Zehnder interferometers (MZIs) 1021, as shown in FIG. 10B.Each MZI may include one input 1023 and two outputs 1033, 1034 of anevanescent coupler 1026 as illustrated in FIG. 10C, or two inputs (oneof which is idle) and two outputs, being two waveguide sections coupledby evanescent couplers at two locations. The function of the MZIA 1020is to split the image light into N portions. In embodiments wherepassive Y-splitters are used, the PSA 1022 may be used to scan acollimated beam. In implementations where active MZIs are used, phaseshifters 1027 in at least one of the two branches of the MZI (FIG. 10C)may be used to control optical power distribution at the outputs to notequal optical powers if required, e.g. to provide apodization of thescanned collimated beam, or even create a full desired 1D angularprofile.

Referring to FIG. 10D, the phase shifter array 1022 may include aplurality of phase shifters 1027 providing a controllable amount ofphase shift, or delay, to light propagating therein. The phase shifters1027 may be, for example, thermo-optic shifters based on thermo-opticeffect, electro-optic shifters based on Pockels and/or Kerr effect,and/or electro-absorption phase shifters based on electro-absorptioneffect in semiconductors, and accordingly may include heaters and/orelectrodes over the waveguides, as required.

Turning to FIG. 10E, the waveguide concentrator 1024 includes an arrayof waveguides fanning in or out to achieve a required output pitch.Typically, the output pitch needs to be small enough to enable largeFOV. The FOV is approximately equal to a ratio of emission wavelength topitch of the output linear waveguides 1030.

Referring to FIG. 11, a hybrid 1D imager 1104 is an exampleimplementation of the 1D redirector 104 of FIG. 1, the 1D imager 204 ofFIG. 2, or the 1D imager 304 of FIG. 3. The hybrid 1D imager 1104 ofFIG. 11 includes a MZIA 1120, a waveguide concentrator 1124 coupled tothe MZIA 1120, and an FOV collimator 1136 coupled to the waveguideconcentrator 1124 implemented in a low-mode waveguide 1106. The MZIA1120 functions as 1×N distributor or switch, where N is the number ofoutput MZIA waveguides 1129. The MZIA 1120 may include, for example, abinary tree of Mach-Zehnder switches for switching image light 1108between N linear output waveguides. The waveguide concentrator 1124brings its output linear waveguides 1130 closer together than the outputMZIA waveguides 1129. Ends of the output linear waveguides 1130 of thewaveguide concentrator 1124 are disposed at a focal plane of the FOVcollimator 1136, which is located in a slab waveguide portion 1107 ofthe few-mode waveguide 1106. The FOV collimator 1136 is a collimatingelement disposed one focal length away from the ends of the outputlinear waveguides 1130 of the waveguide concentrator 1124. The functionof the FOV collimator 1136 is to convert position, or Y-offset, of theends of the output linear waveguides 1130 of the waveguide concentrator1124 into a beam angle of a corresponding output light beam 1119propagating in the slab waveguide portion 1107. In other words, the FOVcollimator 1136 operates as an offset-to-angle optical elementconverting an offset, or the Y-position, of a selected one of the outputwaveguides 1130 carrying a portion of the image light 1108 into an angleof the output light beam 1119 originating from the image light portionpropagated in the selected output waveguide.

The FOV collimator 1136 may be a single element such as lens or mirror,or may include a plurality of lenses 1138, 1140, as shown in FIG. 11.The lenses 1138, 1140 may be formed in the low-mode waveguide 1106 byetching, and may have s folded configuration such as a pancake lensconfiguration, for example. Output light beam 1119 may be reshaped,focused, collimated, etc., in the plane of the waveguide (XY plane),while remaining guided by the slab waveguide portion 1107, i.e. whileremaining constrained in Z-direction. The lens surfaces may include aplurality of tapers 1141 with subwavelength periodicity to facilitate anadiabatic transition between etched and non-etched part of the waveguideand in so doing prevent an out of plane light scattering.

Referring to FIG. 12, a free-space optics (FSO) 1D scanner 1204 includesa microelectromechanical system (MEMS) beam scanner 1244 with a tiltablereflector 1245 and a cylindrical lens 1246. In operation, amulti-wavelength light source, such as a wavelength-swept laser source1202, emits a light beam 1208 at a tunable, e.g. linearly swept,emission wavelength. The light beam 1208 is focused by the cylindricallens 1203 onto the MEMS tiltable reflector 1245. The cylindrical lens1246 functions as a coupler receiving the image light 1208 scanned bythe MEMS beam scanner 1244 and coupling the image light 1208 to alow-mode slab waveguide 1207. The cylindrical lens 1203 may berefractive or diffractive, for example. Other types of couplers such asmirrors, for example, may be used. The MEMS tiltable reflector 1245reflects the light beam 1208 at a variable angle, as shown by adouble-headed arrow. The cylindrical lens 1246 has focusing power in XZplane, while propagating the light beam 1208 substantially withoutfocusing in XY plane. The cylindrical lens 1246 focuses the light beam1208 onto an edge of the low-mode slab waveguide 1207 or onto a gratingcoupler formed in the low-mode slab waveguide 1207. The light beam 1208gets coupled into the low-mode slab waveguide 1207 and propagates freelyin the low-mode slab waveguide 1207 in XY plane, being confined inZ-direction perpendicular to a plane of the low-mode slab waveguide1207. The cylindrical lens 1246 needs to be precisely parallel to thelow-mode slab waveguide 1207 for efficient coupling.

Turning to FIGS. 13A and 13B, a liquid crystal (LC) 1D FOV expander 1324is an embodiment of the 1D FOV expander 224 of FIG. 2 and the 1D FOVexpander 324 of FIG. 3. The LC 1D FOV expander 1324 of FIGS. 13A and 13Bincludes a slab waveguide 1306 having a core 1307 for guiding imagelight 1308 in the core 1307 while allowing the image light 1308 tofreely propagate in XY plane, as depicted. A top cladding 1337 includestunable cladding portions 1339, 1340 evanescently coupled to the core1307 of the slab waveguide 1306 and shaped to deviate light propagatedin the core 1307 of the slab waveguide 1306 by a controllable amount. Inthe embodiment shown in FIG. 13A, the tunable cladding portions 1339,1340 have saw-tooth shapes in XY plane, e.g. an array of triangularshapes, as shown in FIG. 13A. The array extends laterally w.r.t. anoptical path of the image light in the core 1307 of the slab waveguide1306. At least some triangular-shaped tunable cladding portions have aside 1339A, 1340A extending at an acute angle w.r.t. the optical pathrepresented by horizontal arrows in FIG. 13A. An underlayer 1338 (FIG.13B) may be provided under the tunable cladding portions 1339, 1340 toavoid a direct contact between the tunable cladding portions 1339, 1340and the waveguide core 1307. The underlayer 1338 may be thin enough toensure the evanescent coupling of the light propagating in the core 1307with the tunable cladding portions 1339, 1340.

The tunable cladding portions 1339, 1340 of the top cladding 1337include liquid crystal (LC) LC material that changes its refractiveindex for light of a certain polarization upon application of electricfield to the tunable cladding portions 1339, 1140. When refractive indexof the LC tunable cladding portions 1339, 1140 of the top cladding 1337changes, the effective refractive index of the slab waveguide 1306changes as well, causing the image light 1308 to deviate from theoriginal direction of propagation due to a Fresnel refraction on thetilted faces of the LC tunable cladding portions 1339, 1140. Themagnitude of the deviation depends on the angle of the tilted faces andthe vertical pitch (in Y-direction) of the LC tunable cladding portions1339 and 1340. The distribution of energy in directions 1349 and 1350depends on the switching state of the LC tunable cladding portions 1339and 1340, which may be operated in binary mode, ON/OFF. Each array ofthe LC tunable cladding portions 1339 and 1340 may offset the 1D FOV bya discrete amount when the LC tunable cladding portions 1339 and 1340are not be continuously tunable. Therefore, a cascade of m LC elementswould yield 2^(m) combinations of 1D FOV offsets. By energizingdifferent triangular LC tunable cladding portions 1339, 1340, the imagelight 1308 may be deviated at different angles. For example, energizinglarger triangular shapes 1339 deviates the image light 1308 to propagateat an angle as shown at 1349, and energizing shallower triangular shapes1340 deviates the image light 1308 to propagate at a steeper angle, asshown at 1350. More arrays of LC portions with different deviationangles may enable a more precise angle control. When differenttriangular shapes of the LC tunable cladding portions 1339, 1340 areenergized in coordination with operating the 1D imager or scanner, theimage light is switched between a plurality of conterminous FOVportions, enabling effective controllable light spread and associatedhorizontal FOV to be expanded, or enhanced.

Referring to FIG. 14, a holographic beam expander 1426 includes ahologram 1456 in a low-mode slab waveguide 1406 enabling propagation ofimage light in plane of the slab waveguide 1406 in two dimensions, i.e.in XY plane. The hologram 1456 is configured to receive image light 1408including at least one collimated beam portion, e.g. first (1451; dottedlines) and second (1452; dashed lines) collimated beam portionspropagating at an angle to each other, and to reflect each collimatedbeam portion at a plurality of locations along an optical path of thecollimated beam portion in the hologram 1456. For example, the firstcollimated beam portion 1451 is reflected at a plurality of locations1451A, 1451B, 1451C at a first angle, producing a first output beam(1461; dotted lines); and the second collimated beam portion 1452 isreflected at a plurality of locations 1452A, 1452B, 1452C at a secondangle, producing a second output beam (1462; dashed lines). Any otherlight beams are reflected at beam angles between the first and secondangles, propagating in a plurality of directions between the directionsof the first 1461 and second 1462 output beams. To that end, thehologram 1456 may include a plurality of fringes configured to ensurereflection in the desired directions, depending on the impinging beamangles. It is seen from FIG. 14 that such a reflection geometry leads toexpansion of the light beams 1451, 1452.

Turning to FIG. 15, a wide-beam PIC phased array 1D imager 1504 issimilar to the PIC phased array 1D imager 1004 of FIG. 10A, but includesmany more linear waveguides in a MZIA 1520 and a PSA 1522 (FIG. 15). Anoutput waveguide array coupler or beam expander 1524 is similar to theconcentrator 1024, but includes many more output linear waveguides 1530,e.g. 10,000; 20,000; 30,000 or more waveguides spanning over 5 mm; 10mm; or 15 mm of lateral distance, as the case may be. The output beammay be as wide as the width of the linear waveguide 1530 array, and thusmay not require any subsequent beam expansion and collimation to expandthe output beam over an eyebox of a near-eye display, simplifying theoverall design.

Referring now to FIG. 16, a slab waveguide 1606 supports a diffractiongrating 1661 for diffracting out a portion of image light 1608propagating in the XY plane of the slab waveguide 1606. A K-vector ofthe diffraction grating 1661 is aligned with the x-component of theK-vector of the image light 1608. A diffraction angle θ of the imagelight 1608 follows the equation

$\begin{matrix}{{\sin\theta} = {n - \frac{\lambda}{T}}} & (1)\end{matrix}$

where n is refractive index, T is grating period and λ is the wavelengthof light in the propagation medium. Accordingly,

$\begin{matrix}{\frac{d\;\theta}{d\;\lambda} = {{\left( \frac{1}{\cos(\theta)} \right)*\left( {\frac{dn}{d\;\lambda} - \frac{1}{T}} \right)} = {\left( \frac{1}{\cos\;(\theta)} \right)*\left( {\frac{n - n_{g\; r}}{d\;\lambda} - \frac{1}{T}} \right)}}} & (2)\end{matrix}$

where n_(gr) is a group index. In a regular waveguide with smalldispersion, n_(gr)=n; therefore,

$\begin{matrix}{\frac{d\;\theta}{d\lambda} = \frac{1}{T*{\cos(\theta)}}} & (3)\end{matrix}$

It follows from Eq. (3) that at normal incidence and T=λ/n that

$\begin{matrix}{{d\;\theta} = {\frac{n*d\;\lambda}{\lambda} = {4.4{^\circ}}}} & (4)\end{matrix}$

for n=2 at wavelengths ranging from 510 nm to 530 nm. In accordance withthis disclosure, the angular range may be increased to some extent byincreasing the angle of diffraction θ. For example, by reducing thegrating pitch to 180 nm, one may achieve the range of diffraction anglesdθ to 14.4 degrees.

The angular dispersion range may be increased e.g. by using amulti-layer output waveguide. Referring to FIG. 17, a dual-core slabwaveguide 1706 extends in XY plane. The dual-core slab waveguide 1706includes first 1707 and second 1757 cores running parallel to oneanother in XY plane of a substrate 1736 surrounded by first 1705 andsecond 1755 claddings respectively. The first core 1707 and the firstcladding 1705 are configured for singlemode propagation of a first beam(solid arrows; 1708) of image light. Similarly, the second core 1757 andthe second cladding 1755 are configured for singlemode propagation of asecond beam (dashed arrows; 1758) of image light.

The first 1707 and second 1757 cores have first 1710 and second 1760diffraction gratings formed in or on the first 1707 and second 1757cores, respectively. The first diffraction grating 1710 is configured toout-couple spectral component(s) of the redirected image light at afirst angle dependent on the first wavelength. The first angle is withina first angle range corresponding to a tuning range of awavelength-tunable light source used. Similarly, the second diffractiongrating 1760 is configured to out-couple spectral component(s) of theredirected image light at a second angle different from the first angle.The second angle is within a second angle range corresponding to thetuning range of the wavelength-tunable light source.

Different angles and angular ranges of diffraction from different cores1707, 1757 of the dual-core waveguide 1706 can be attained by changingthe thickness or refractive index of the cores 1707 and 1757, refractiveindex of the claddings 1705 and 1755, or pitch of the diffractiongratings 1710 and 1760, for the same wavelength of the image lightrepresented by the first 1708 and second 1758 light beams. To simplifyfabrication, a single grating can be etched into the first core 1707layer, and the second core 1757 layer or any subsequent layer(s) mayreproduce this grating simply through a directional material deposition.In the latter case, the FOV may be adjusted by varying the thickness ofthe layers and, therefore, the effective refractive index.

Different angles of diffraction may be used to expand the corresponding“vertical” 1D FOV by tiling smaller angular ranges from separate layers.Herein, the term “vertical” is meant to differentiate from the 1D FOV byredirecting the image light in plane of the low-mode waveguide, i.e. XYplane, which is termed “horizontal”. It is to be noted that the terms“horizontal” and “vertical” in this context are meant as meredifferentiators to distinguish in-plane 1D FOV from wavelengthdispersion 1D FOV, and do not imply the actual orientation of thedevices when in use. The switching between the first 1707 and second1757 cores can be achieved, for example, using Mach-Zehnderinterferometers and directional couplers. More details on possibleswitching configurations will be provided further below.

Turning to FIG. 18, an angular dispersion module 1828 provides enhancedwavelength dispersion of image light to obtain the required vertical 1DFOV. The angular dispersion module 1828 includes serially coupled: aMZIA 1820 coupled to a vertical mode converter 1821, e.g. anon-symmetrical directional coupler, a multimode interference (MMI)coupler 1851 receiving light from the vertical mode converter 1821, anda few-mode slab waveguide portion 1856 receiving light from the MMIcoupler 1851. The number of transversal modes of propagation of imagelight that may propagate in a core of the few-mode slab waveguideportion 1956 may be 2, 3, 4, 5, or 6, for example, or more generally nomore than 10 modes. A diffraction grating 1807 disposed in or on thecore of the few-mode slab waveguide portion 1856 operates as anout-coupler, out-coupling the image light at an angle dependent onwavelength. The diffraction grating 1807 is configured to out-coupleeach spectral component of the redirected image light at an angledependent on the wavelength of the spectral component. The out-couplingangle is within an out-coupling angle range corresponding to a tuningrange of the wavelength-tunable light source used. The out-couplingangle range is different for different lateral modes of propagation ofthe few-mode slab waveguide portion 1856, because each mode ofpropagation has a different effective refractive index.

Initially, only a fundamental mode of image light is coupled by anin-coupler 1803 into the MZIA 1820. The MZIA 1820 functions as 1×Noptical switch, switching the image light between its output waveguides.At the end of each MZIA 1820 output waveguide, the image light isconverted into a different vertical mode by the vertical mode converter1821. The image light from all the waveguides is combined into thefew-mode slab waveguide portion 1956 using the MIMI coupler 18511 n thismanner, the in-plane image encoder layout, i.e. horizontal 1D imagercircuitry disclosed above with reference to FIGS. 9-15, may be sharedbetween different modes of propagation of image light in the FMW 1856.

Different vertical modes have different effective refractive indicesand, therefore, will diffract at the diffraction grating 1807 atdifferent angles for the same wavelength. The overall vertical 1D FOVmay be expanded using diffraction ranges of the separate modes in atime-sequential manner, that is, switching to a particular coreproviding a corresponding vertical 1D FOV portion, then switching toanother core providing a different vertical 1D FOV portion, and so on,until all the vertical 1D FOV is covered. The MMI coupler 1851 may beoptimized for the required coupling of vertical modes using a physicaldesign software by defining an optimization function (also termed meritfunction) to have operands representing optical insertion loss of theMMI coupler for each vertical mode having pre-defined verticalcoordinates, and letting the physical design software run theoptimization.

The coupling configuration of FIG. 18, i.e. the MZIA 182 coupled to thevertical mode converter 1821 coupled to the MIMI 1851, may be used tocouple the image light received by the in-coupler 1803 into differentvertical modes of any waveguide component supporting several verticalmodes. Such a configuration may be used, for example, to couple imagelight into cores of the dual-core slab waveguide 1706 of FIG. 17. Incase of the dual-core slab waveguide 1706, only one Mach-Zehnderinterferometer, operating as 1×2 optical switch, may be used instead ofthe MZIA 1820. A multi-core slab waveguide may include a plurality ofcores, and the coupling configuration of FIG. 18 may be used to couplethe image light into any of the cores of the multi-core slab waveguide.

Referring to FIG. 19, an angular dispersion enhancer 1928 is based on aslow-light slab waveguide 1906 extending in XY plane and supporting acladding 1905 with a diffraction grating structure 1910 for out-couplingimage light form the slow-light waveguide 1906. The slow-light waveguide1906 may include a uniform 2D photonic crystal, a multilayer structure,or a combination of the two, for increasing a group velocity refractiveindex by a factor of 20, for example, or at least by a factor of 10.Then, it follows from Eqs. (2)-(4) than the FOV may be increasedtenfold, e.g. from 4.4 degrees to 44 degrees for the slowing factor of20. In some embodiments, the slow-light waveguide 2006 may include anarray of linear photonic crystal waveguides.

Referring to FIG. 20A, an angular dispersion enhancer 2028A includes alow-mode waveguide 2006 having a slab waveguide portion 2056 disposed inXY plane, and a corrugated reflector 2070A supported by the slabwaveguide portion 2056. Image light 2008, carrying image informationencoded in its spectrum, propagates in the slab waveguide portion 2056.A diffraction grating 2010, e.g. a Bragg grating, in the slab waveguideportion 2056 out-couples different spectral components of image light2008 at different angles exceeding 90 degrees w.r.t. a direction ofpropagation of the image light 2008 in the slab waveguide portion 2056.For example, a first spectral component 2081 is out-coupled at a firstangle θ₁, and a second spectral component 2081 is out-coupled at asecond, larger angle θ₂. The corrugated reflector 2070A reflects thefirst 2081 and second 2082 spectral components of the image light 2008diffracted by the diffraction grating 2010 through the slab waveguideportion 2056 and outside of the low-mode waveguide 2056.

The angular dispersion of the out-coupled image light 2008 is largestwhen the image light 2008 is out-coupled almost directly back, i.e. theangle θ approaches 180 degrees. For example, assuming a regularwaveguide grating with refractive index of 2, changing the wavelengthfrom 510 nm to 530 nm causes 4.4° shift if the averaged diffractionangle θ is zero, but 14° shift if the average diffraction angle θ is˜65°. In this configuration, the diffraction grating 2010 out-couplesthe image light 2008 backwards to maximize the angular dispersion and,therefore, increase the FOV of a display. The corrugated reflector 2070Amay include a plurality of prisms 2072 with reflective coating 2074supported by the slab waveguide portion 2056, which redirect the imagelight 2008 in the direction normal to the slab waveguide portion 2056 tomake sure that the central field angle is perpendicular to the slabwaveguide portion 2056, i.e. is parallel to Z-axis. By way of anon-limiting example, the reflective layer 2074 can be made of eitherone of the following or a combination of: (1) a lower refractive indexmaterial for total internal reflection (TIR), (2) a thin metal layerforming a semi-reflective mirror, or (3) a narrow-spectrum multilayermirror coating or a reflective polarizer, such as a wire grid polarizeror a dual brightness enhancement film (DBEF).

Turning to FIG. 20B, an angular dispersion enhancer 2028B is apolarization-selective embodiment of the angular dispersion enhancer2028A of FIG. 20A. In the angular dispersion enhancer 2028B of FIG. 20B,the corrugated reflector includes a polarization-selective reflector2070B configured to reflect light at a first polarization and transmitlight at a second polarization orthogonal to the first polarization. Theangular dispersion enhancer 2028B further includes a quarter-wavewaveplate (QWP) 2076 supported by the slab waveguide portion 2056 on anopposite side of the slab waveguide portion 2056 from thepolarization-selective reflector 2070B. The QWP 2076 is configured toreceive the image light components 2081, 2082 reflected by thepolarization-selective reflector 2070B. The image light components 2081,2082 are in the first polarization state.

A diffractive structure 2078, e.g. a reflective surface-reliefdiffraction grating, is supported by the QWP 2076 and configured toreflect the image light components 2081, 2082 propagated through the QWP2076 back to propagate through the QWP 2076 for a second time convertingthe polarization of the image light components 2081, 2082 from the firstpolarization to the second polarization. Then, the components 2081, 2082propagate through the slab waveguide portion 2056, and through thepolarization-selective reflector 2070B, which transmits them throughbecause they are in the second polarization state. The purpose of thediffractive structure 2078 is to further increase the angular dispersionof the image light 2008.

Referring to FIGS. 21A and 21B, a principle of varifocal adjustment(i.e. adjusting the convergence/divergence) of the out-coupled imagelight is illustrated. A low-mode slab waveguide portion 2156A in FIG.21A includes a grating out-coupler with a uniform refractive index. Animage light component 2108 is out-coupled vertically, i.e. perpendicularto the plane of the slab waveguide portion 2156A, as a parallel beamfocused at infinity. A slab waveguide portion 2156B in FIG. 21B includesa grating out-coupler with a controllable non-uniform refractive indexto cause the image light component 2108 to be out-coupled vertically,i.e. perpendicular to the plane of the slab waveguide portion 2156B,with a convergence as shown, as a focused beam at a focal distance equalto an inverse of the optical power (focusing power) of the gratingout-coupler. Assuming a desired size of an eyebox 2112 of 16 mm, a 0.008refractive delta is required to focus the image light component 2108from the slab waveguide portion 2156B into a focal spot 2185 two metersaway for the eyebox 2112. To achieve this focusing function, theeffective refractive index n_(eff) of the grating out-coupler needs tovary linearly from a 0.004 to −0.004 in going across the eyebox 2112.This principle may be used in any dispersion enhancer considered herein.For a dispersion enhancer with a resonating structure, any change in thephysical refractive index of the materials in the stack would generate alarger change in n_(eff), thus facilitating the focusing or defocusingdue to the change of refractive index along an out-coupler. Illustrativeexample of varifocal adjusters based on this principle are consideredbelow with reference to FIGS. 22 to 25.

Referring first to FIG. 22, a varifocal adjuster 2230 includes alow-mode slab waveguide portion 2256 for propagating light, e.g. imagelight 2208. The slab waveguide portion 2256 includes an out-coupler2210, e.g. a Brag grating, configured to out-couple the image light 2208at an angle to the XY plane of the slab waveguide portion 2256. A liquidcrystal (LC) cell 2288 is evanescently coupled to the slab waveguideportion 2256. The thickness of an upper cladding 2287 of the slabwaveguide portion 2256 is selected such that a tail of a guided mode2290 of the image light 2208 travelling in the core of the mode slabwaveguide portion 2256 overlaps the LC cell 2288, reaching an LC layer2289 of the LC cell 2288 through an upper cladding 2287 supporting theLC cell 2288. The LC layer 2289 is disposed between a pair of electrodes2283. The guided mode 2290 overlaps the LC layer 2289 of the LC cell2288.

The LC cell 2288 defines an effective refractive index for the guidedmode 2290 of the image light 2208 propagating in the slab waveguideportion 2256. The effective refractive index n_(eff) varies in adirection of propagation of the image light 2208 in the low-mode slabwaveguide 2256, that is, X-direction in FIG. 22. The varying effectiverefractive index n_(eff) causes a direction of the out-coupled imagelight 2208 portions to vary in going along X-axis, which causes theout-coupled image light 2208 to focus or defocus, as explained abovewith reference to FIGS. 21A and 21B. The thickness profile of the uppercladding 2287 (FIG. 22) may be selected so that a change of therefractive index of the LC layer 2289 would cause a linear change of theeffective refractive index n_(eff) for the propagating waveguide mode2290. This will cause the out-coupled image light 2208 to be focused ordefocused. The absolute value of the change and, therefore, the focallength, can be controlled by a voltage applied to the LC cell 2288. Thethickness profile of the upper cladding 2287 may be made linear, i.e.the LC cell 2288 may form an acute angle with the slab waveguide portion2256, thereby varying the effective refractive index n_(eff) for thewaveguide mode 2290.

The refractive index of the LC layer 2289 is varied by applying voltageto the LC cell 2288. As explained above, this causes a direction ofpropagation of the out-coupled image light 2208 to vary along thedirection of propagation (i.e. X-direction) of the image light 2208 inthe slab waveguide portion 2256, causing the out-coupled image light2208 to diverge or converge in XZ plane. By varying the applied voltage,the divergence/convergence of the out-coupled image light 2208(collectively termed “divergence”) may controlled. In some embodiments,the LC cell 2288 may be parallel to the slab waveguide portion 2256, andmay be pixelated to impart a refractive index change profile along thedirection of propagation of the image light 2208, that is, along theX-direction.

Referring now to FIG. 23, a varifocal adjuster 2330 includes a low-modeslab waveguide portion 2356 for propagating light, e.g. image light2308. The slab waveguide portion 2356 includes a core layer 2307 and anout-coupler 2310 configured to out-couple the image light 2308 at anangle to a plane of the slab waveguide portion. The core layer 2307 ismade of a material having a refractive index dependent on an appliedelectric field. For example, the core layer 2307 may be made of LiNbO3,AlN, SiC, or another material with a high electro-optic coefficient.

Electrodes 2383 are disposed above and below the core layer 2307 forapplying an electric field 2386 to the core layer 2307. The electrodes2383 may be disposed at an acute angle to each other, forming a wedge.When a voltage is applied to the electrodes 2383, the electric field2386 spatially varies along the direction of propagation of the imagelight 2308 in the core 2307 of the slab waveguide portion 2356, i.e.along X-direction. This causes a direction of the image light 2308out-coupled from the slab waveguide portion 2356 to vary along the adirection of propagation of the light in the low-mode slab waveguide,effectively causing the out-coupled image light 2208 to diverge orconverge in XZ plane. By varying the applied voltage, the degree ofconvergence/divergence of the out-coupled image light 2308 may be variedin a controllable manner.

The spatial modulation of refractive index may be achieved by a DC or ACelectric field that passes through the materials of the slab waveguideportion 2356 and a propagating optical mode 2390. Depending on thecrystal axis in which the refractive modulation is desired and thecomponent design, electrodes can be placed either above/below theupper/lower cladding respectively, or just in one of these layers. Incase where the electrodes sandwich the core of the waveguide, theelectric field 2386 will run vertically as shown in FIG. 23. Therelative magnitude of the electric field 2386 along the propagation ofthe image light 2308 (which is the one that needed to be controlled toachieve focusing) can be assigned by changing the distance betweenelectrodes 2382. Larger distance will yield a weaker electric field2386. By doing so, one can embed a pre-determine wedge profile electricfield 2386 that proportionally correlates with local refractive indexchange. When the applied voltage V=0, the image light 2308 iscollimated, i.e. the image is at infinity. As the applied voltage V isincreased, the focal plane of the system will get closer.

A similar principle is applied in a varifocal adjuster 2430 of FIG. 24,where an electric field 2486 is parallel to a core layer 2407 of afew-mode slab waveguide 2456. Such orientation of the electric field2486 is defined by floating electrodes 2482 extending along a directionof propagation of the image light 2408 in the low-mode slab waveguide2456 between a pair of end electrodes 2485, to which the voltage V maybe applied. The floating electrodes 2482 are disposed so as to provide amagnitude distribution of the electric field 2486 that matches arefractive index distribution required for focusing of the out-coupledimage light 2408. In some embodiments, a set of independently controlledelectrodes might be provided to have a better control of the magnitudedistribution of the electric field 2486.

Referring now to FIG. 25, a varifocal adjuster 2530 includes a low-modeslab waveguide portion 2506 supporting a grating structure 2510including an array of grating fringes 2511 having a first refractiveindex and surrounded by a substrate 2512 between individual gratingfringes 2510 having a second refractive index. The grating structure2510 out-couples image light 2508 form the low-mode slab waveguideportion 2506. At least one of the first or second refractive index istunable to provide a gradient of the at least one of the first or secondrefractive index for focusing or defocusing of the image light 2508out-coupled from the slab waveguide portion by the out-coupler. To thatend, an array of heating elements 2570 may be coupled to the waveguide2506 for providing a non-uniform, spatially selective heating to thegrating structure 2510. The spatially selective heating creates arefractive index gradient, which may modify local diffraction angle forthe image light 2508, thereby enabling focusing or defocusing of theimage light 2508 out-coupled from the waveguide 2506. By way of anon-limiting example, to achieve a 1 m focal distance for 16 mm eyeboxlength one needs a maximum Δn=0.008 for the effective refractive index.This number will become proportionally smaller if a slow light waveguideis used instead of the waveguide 2506. In some embodiments, thesubstrate 2510 may include a liquid crystal (LC) layer to provide arequired refractive index gradient by tuning the LC layer with anapplied electric field. Since the layer in which the refractive index ismodified is thin, it will only affect the display optical path but notthe see-through optical path.

Referring to FIG. 26 with further reference to FIG. 1A, a method 2600(FIG. 26) for providing an image in angular domain includes coupling(2602) image light, e.g. the light 108 in FIG. 1A, including a spectralcomponent at a first wavelength, to a 1D redirector/imager using anin-coupler, e.g. the in-coupler 103. The image light may be redirectedusing any of the horizontal FOV 1D redirectors/imagers disclosed herein,e.g. the phased array 1D imager 904 of FIG. 9 including any of the PICembodiments of FIGS. 10A to 10E, the hybrid 1D imager 1104 of FIG. 11,or the FSO 1D scanner 1204 of FIG. 12. The 1D imager redirects (2604)the image light in a first plane, e.g. in the XY plane (which is theplane of the low-mode slab waveguide portion 107) in FIG. 1A. The imagelight 108 redirected by the 1D imager is propagated (2606) tin a in alow-mode slab waveguide portion 107. The spectral component at the firstwavelength is out-coupled (2608) from the low-mode slab waveguide by theout-coupler 110, e.g. any of the grating out-couplers considered herein,at an angle dependent on the first wavelength. The low-mode slabwaveguide portion may include a singlemode slab waveguide or a few-mode(no greater than 10 modes) slab waveguide.

Turning to FIG. 27, a method 2700 is an embodiment of the method 2600 ofFIG. 26. The method 2700 of FIG. 27 uses a monochromatic tunable sourceof image light, such as the tunable laser source 502 of FIG. 5, forexample. The method 2700 includes setting (2702) an emission wavelengthof the monochromatic tunable source and redirecting (2704) the imagelight coupled into the 1D imager in the XY plane. In other words, thewavelength of the tunable light source is not shifted while the 1Dimager redirects the image light in the first plane, whereby the imagelight redirected in the first plane is out-coupled at the same angle.Then, next wavelength (2706) is set and the process repeats, at its ownvalue of output power of the light source corresponding to a desiredbrightness of a pixel of an image being displayed. The redirection in XYplane includes angularly scanning (2708) a collimated light beam in XYplane, or forming the angular distribution of brightness simultaneously(2710).

Referring now to FIG. 28, a method 2800 is an embodiment of the method2600 of FIG. 26. The method 2800 of FIG. 28 uses a tunable-spectrumlight source, e.g. the tunable-spectrum light source 602 of FIG. 6A. Themethod 2800 of FIG. 28 includes using the tunable-spectrum light sourceto provide (2802) image light having a plurality of spectral componentscorresponding to the vertical FOV of the image to be displayed. Theimage light with the plurality of spectral components is redirected(2804) in XY plane by providing angular distribution of brightness in XYplane, which may be done by scanning (2808) or forming the instantaneousangular distribution (2810). The angularly dispersed, multi-wavelengthimage light is out-coupled (2806) from the low-mode slab waveguide withan angular distribution corresponding to the spectral composition of theimage light.

Turning to FIG. 29, an augmented reality (AR) near-eye display 2900includes a frame 2901 having a form factor of a pair of eyeglasses. Theframe 2901 supports, for each eye, a light engine 2908 including atunable-spectrum light source described herein, and a low-mode, i.e. asinglemode or few-mode waveguide 2910 disclosed herein, opticallycoupled to the light engine 2908. The AR near-eye display 2900 mayfurther include an eye-tracking camera 2904, a plurality of illuminators2906, and an eye-tracking camera controller 2907. The illuminators 2906may be supported by the waveguide 2910 for illuminating an eyebox 2912.The light engine 2908 provides a light beam having a spectrumrepresentative of the vertical 1D FOV to be projected into a user's eye.The waveguide 2910 receives the light beam and expands the light beamover the eyebox 2912. The horizontal 1D FOV may be provided by a 1Dimager disclosed herein, e.g. the PIC-based imager 436 of FIG. 4A, thephased array 1D imager 904 of FIG. 9, the hybrid 1D imager 1104 of FIG.1, or MEMS-based scanner of 1204 FIG. 12.

The purpose of the eye-tracking cameras 2904 is to determine positionand/or orientation of both eyes of the user. Once the position andorientation of the user's eyes are known, a gaze convergence distanceand direction may be determined. The imagery displayed may be adjusteddynamically to account for the user's gaze, for a better fidelity ofimmersion of the user into the displayed augmented reality scenery,and/or to provide specific functions of interaction with the augmentedreality. In operation, the illuminators 2906 illuminate the eyes at thecorresponding eyeboxes 2912, to enable the eye-tracking cameras toobtain the images of the eyes, as well as to provide referencereflections i.e. glints. The glints may function as reference points inthe captured eye image, facilitating the eye gazing directiondetermination by determining position of the eye pupil images relativeto the glints images. To avoid distracting the user with illuminatinglight, the latter may be made invisible to the user. For example,infrared light may be used to illuminate the eyeboxes 2912.

The function of the eye-tracking camera controllers 2907 is to processimages obtained by the eye-tracking cameras 2904 to determine, in realtime, the eye gazing directions of both eyes of the user. In someembodiments, the image processing and eye position/orientationdetermination functions may be performed by a central controller, notshown, of the AR near-eye display 2900. The central controller may alsoprovide control signals to the light engines 2908 depending on thedetermined eye positions, eye orientations, gaze directions, eyesvergence, etc.

Referring now to FIG. 30, an HMD 3000 is an example of an AR/VR wearabledisplay system which encloses the user's face, for a greater degree ofimmersion into the AR/VR environment. The function of the HMD 3000 is toaugment views of a physical, real-world environment withcomputer-generated imagery, or to generate the entirely virtual 3Dimagery. The HMD 3000 may include a front body 3002 and a band 3004. Thefront body 3002 is configured for placement in front of eyes of a userin a reliable and comfortable manner, and the band 3004 may be stretchedto secure the front body 3002 on the user's head. A display system 3080may be disposed in the front body 3002 for presenting AR/VR imagery tothe user. Sides 3006 of the front body 3002 may be opaque ortransparent.

In some embodiments, the front body 3002 includes locators 3008 and aninertial measurement unit (IMU) 3010 for tracking acceleration of theHMD 3000, and position sensors 3012 for tracking position of the HMD3000. The IMU 3010 is an electronic device that generates dataindicating a position of the HMD 3000 based on measurement signalsreceived from one or more of position sensors 3012, which generate oneor more measurement signals in response to motion of the HMD 3000.Examples of position sensors 3012 include: one or more accelerometers,one or more gyroscopes, one or more magnetometers, another suitable typeof sensor that detects motion, a type of sensor used for errorcorrection of the IMU 3010, or some combination thereof. The positionsensors 3012 may be located external to the IMU 3010, internal to theIMU 3010, or some combination thereof.

The locators 3008 are traced by an external imaging device of a virtualreality system, such that the virtual reality system can track thelocation and orientation of the entire HMD 3000. Information generatedby the IMU 3010 and the position sensors 3012 may be compared with theposition and orientation obtained by tracking the locators 3008, forimproved tracking accuracy of position and orientation of the HMD 3000.Accurate position and orientation is important for presentingappropriate virtual scenery to the user as the latter moves and turns in3D space.

The HMD 3000 may further include a depth camera assembly (DCA) 3011,which captures data describing depth information of a local areasurrounding some or all of the HMD 3000. To that end, the DCA 3011 mayinclude a laser radar (LIDAR), or a similar device. The depthinformation may be compared with the information from the IMU 3010, forbetter accuracy of determination of position and orientation of the HMD3000 in 3D space.

The HMD 3000 may further include an eye tracking system 3014 fordetermining orientation and position of user's eyes in real time. Theobtained position and orientation of the eyes also allows the HMD 3000to determine the gaze direction of the user and to adjust the imagegenerated by the display system 3080 accordingly. In one embodiment, thevergence, that is, the convergence angle of the user's eyes gaze, isdetermined. The determined gaze direction and vergence angle may also beused for real-time compensation of visual artifacts dependent on theangle of view and eye position. Furthermore, the determined vergence andgaze angles may be used for interaction with the user, highlightingobjects, bringing objects to the foreground, creating additional objectsor pointers, etc. An audio system may also be provided including e.g. aset of small speakers built into the front body 3002.

Embodiments of the present disclosure may include, or be implemented inconjunction with, an artificial reality system. An artificial realitysystem adjusts sensory information about outside world obtained throughthe senses such as visual information, audio, touch (somatosensation)information, acceleration, balance, etc., in some manner beforepresentation to a user. By way of non-limiting examples, artificialreality may include virtual reality (VR), augmented reality (AR), mixedreality (MR), hybrid reality, or some combination and/or derivativesthereof. Artificial reality content may include entirely generatedcontent or generated content combined with captured (e.g., real-world)content. The artificial reality content may include video, audio,somatic or haptic feedback, or some combination thereof. Any of thiscontent may be presented in a single channel or in multiple channels,such as in a stereo video that produces a three-dimensional effect tothe viewer. Furthermore, in some embodiments, artificial reality mayalso be associated with applications, products, accessories, services,or some combination thereof, that are used to, for example, createcontent in artificial reality and/or are otherwise used in (e.g.,perform activities in) artificial reality. The artificial reality systemthat provides the artificial reality content may be implemented onvarious platforms, including a wearable display such as an HMD connectedto a host computer system, a standalone HMD, a near-eye display having aform factor of eyeglasses, a mobile device or computing system, or anyother hardware platform capable of providing artificial reality contentto one or more viewers.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. A display device for providing a line of an imagein angular domain, the display device comprising: a wavelength-tunablelight source for providing image light comprising a spectral componentat a first wavelength; and a low-mode waveguide supporting no more than10 lateral modes of propagation, the low-mode waveguide comprising: anin-coupler for coupling the image light into the low-mode waveguide; anda slab waveguide portion for propagating the image light coupled by thein-coupler; wherein the slab waveguide portion comprises an out-couplerconfigured to out-couple the spectral component of the image light at anangle to a plane of the slab waveguide portion, wherein the angle isdependent on the first wavelength.
 2. The display device of claim 1,wherein the slab waveguide portion comprises a singlemode slabwaveguide.
 3. The display device of claim 1, wherein the slab waveguideportion comprises a core and a top cladding supported by the core,wherein the in-coupler comprises a diffraction grating formed in thecore of the slab waveguide portion.
 4. The display device of claim 1,wherein the slab waveguide portion comprises a first core and a firstcladding supported by the first core, the first core and the firstcladding configured for singlemode propagation of the image light, theout-coupler comprising a first diffraction grating formed in the firstcore; wherein the first diffraction grating is configured to out-couplethe spectral component of the redirected image light at a first angledependent on the first wavelength, wherein the first angle is within afirst angle range corresponding to a tuning range of thewavelength-tunable light source.
 5. The display device of claim 4,wherein the slab waveguide portion further comprises a second coresupported by the first cladding and a second cladding supported by thesecond core, the second core and the second cladding configured forsinglemode propagation of the image light, the out-coupler furthercomprising a second diffraction grating formed in the second core;wherein the second diffraction grating is configured to out-couple thespectral component of the redirected image light at a second angledifferent from the first angle, wherein the second angle is within asecond angle range corresponding to the tuning range of thewavelength-tunable light source, wherein the second range is differentfrom the first range.
 6. The display device of claim 5, furthercomprising a multimode interference (MMI) coupler in an optical pathbetween the in-coupler and the first and second cores of the slabwaveguide portion, for coupling the image light into at least one of thefirst and second cores of the slab waveguide portion.
 7. The displaydevice of claim 6, further comprising a 1×2 optical switch and avertical mode converter downstream of the 1×2 optical switch in anoptical path between the in-coupler and the MMI coupler; wherein aninput port of the 1×2 optical switch is coupled to the in-coupler, andfirst and second output ports of the 1×2 optical switch are coupled tofirst and second input ports, respectively, of the vertical modeconverter; and wherein the vertical mode converter is configured tocouple light at its first input port to the first core of the slabwaveguide portion, and couple light at its second input port to thesecond core of the slab waveguide portion.
 8. The display device ofclaim 1, further comprising a focusing grating supported by the slabwaveguide portion, the focusing grating comprising: an array of gratingfringes having a first refractive index; and a substrate betweenindividual fringes of the array of grating fringes, the substrate havinga second refractive index; wherein at least one of the first or secondrefractive index is tunable to provide a gradient of the at least one ofthe first or second refractive index for focusing or defocusing of theimage light out-coupled from the slab waveguide portion by theout-coupler.
 9. The display device of claim 8, wherein the substratecomprises liquid crystals.
 10. The display device of claim 8, furthercomprising a spatially selective heater coupled to the focusing gratingand configured to create the temperature gradient across the focusinggrating to provide a gradient of the at least one of the first or secondrefractive index.
 11. The display device of claim 1, wherein the slabwaveguide portion comprises a photonic crystal slab layer supporting acladding layer and having a group index of at least 10, wherein theout-coupler comprises a diffraction grating supported by the claddinglayer for out-coupling the image light propagating in the photoniccrystal slab layer.
 12. The display device of claim 1, wherein the slabwaveguide portion comprises a few-mode slab waveguide supporting no morethan 10 lateral modes of propagation, the few-mode slab waveguidecomprising a core, wherein the out-coupler comprising a diffractiongrating formed in or on the core; wherein the diffraction grating isconfigured to out-couple the spectral component of the redirected imagelight at an angle dependent on the first wavelength, wherein the angleis within an angle range corresponding to a tuning range of thewavelength-tunable light source, wherein the angle range is differentfor different lateral modes of propagation of the few-mode slabwaveguide.
 13. The display device of claim 12, further comprising amultimode interference (MMI) coupler in an optical path between thein-coupler and the few-mode slab waveguide portion, for coupling theimage light into at least one mode of propagation of the few-mode slabwaveguide.
 14. The display device of claim 13, further comprising a 1×Noptical switch and a vertical mode converter downstream of the 1×Noptical switch in an optical path between the in-coupler and the MMIcoupler; wherein an input port of the 1×N optical switch is coupled tothe in-coupler, and N output ports of the 1×N optical switch are eachcoupled to a particular one of N input ports, respectively, of thevertical mode converter, wherein N is an integer; and wherein thevertical mode converter is configured to couple light received at itsinput ports to a corresponding mode of propagation of the few-mode slabwaveguide portion.
 15. The display device of claim 1, wherein theout-coupler comprises a diffraction grating configured to out-couple thespectral component of the image light at an angle exceeding 90 degreesw.r.t. a direction of propagation of the image light in the slabwaveguide portion, the device further comprising a corrugated reflectorsupported by the slab waveguide portion for reflecting the image lightdiffracted by the diffraction grating through the slab waveguide portionand outside of the low-mode waveguide.
 16. The display device of claim15, wherein the corrugated reflector comprises a polarization-selectivereflector configured to reflect light at a first polarization andtransmit light at a second polarization orthogonal to the firstpolarization, the device further comprising: a quarter-wave waveplate(QWP) supported by the slab waveguide portion on an opposite side of theslab waveguide portion from the polarization-selective reflector, andconfigured for receiving the image light reflected by thepolarization-selective reflector, the image light having the firstpolarization; and a diffractive structure supported by the quarter-wavewaveplate and configured to reflect the image light propagated throughthe QWP back to propagate through the QWP for a second time convertingthe polarization of the image light to the second polarization, throughthe slab waveguide portion, and through the polarization-selectivereflector.